Cheapo mains-to-DC converters

[Timothy Murphy]

I bought a cheap power converter (Chinese of course) at my local pound shop. It looks very pretty, and claims to output 6 voltages, from 1.5 to 12 volts, with a switch to choose between them, and also has 4 output jacks, plus an output to charge a 9v rechargeable battery.

However, whatever voltage I choose my multimeter says the output is 18 volts.

Could this actually change to the required voltage when there is a load?

Is it worth buying another, of the same type, to see if it works any better?

[Ian Stirling]

Perhaps.

A kettle will have a resistance of about 24 ohms, and will draw about 1/2A at

12V. Connect the live and neutral to the + and - and see if there is a difference.

Also, it may be that it's a very high resistance to 18V - try licking your fingers and putting them across the outputs.

However, I'd be reluctant to use it for anything with electronics in.

[s--p--o--n--i--x]

The voltages will drop when the supply is under load.

A regulated supply will give a steadier voltage but will cost more money.

For most things an unregulated supply will be possibly be sufficient as a lot of equipment contains internal regulators.

[Dave Plowman (News)]

If it's unregulated, then the voltage selector is probably just a series of resistors, and your high impedance DVM is ignoring them.

Use a load resistor based on the maximum current output and ohms law and check again.

18 volts is quite likely from an open circuit 12 volt supply.

[bigcat]

Normally these things have a slide switch to 4 transformer tappings for different Vs. This ac v is rectified and stored in the reservoir cap.

There are a few issues:

once the cap is charged to 18v, there it stays. Moving the switch makes no difference, nor does unplugging it. Solution is short the output with power off.

2nd regulation of those things is real poor. Offload v is around 1.5v on load v.

3rd if lightly loaded, Vout will be too high.

They will power tronic equipment fine if you take these precautions:

plug them into the equipment _before_ switching mains on.

if you select a higher v first by mistake, short the output before connecting.

if youre running a minimal load, select the v switch one stop lower.

Lots of kit uses these kind of things.

NT